This invention relates to computer systems and, more particularly, to methods and apparatus for controlling the output voltage produced by charge pumps used for programming and erasing flash electrically-erasable programmable read only memory (flash EEPROM) arrays.
There has been a recent trend to reducing the power requirements of portable computers. In order to lower power consumption, much of the integrated circuitry used in personal computers is being redesigned to function at lower voltage levels. Some of the circuitry used in portable computers are being designed to operate at low voltage levels such as 5 volts and 3.3 volts. This helps to reduce the power needs of such computers.
Unfortunately, some devices used in portable computers require higher voltages. Flash electrically erasable programmable read only memory (flash EEPROM memory) has recently been used to store the basic input/output startup (BIOS) processes for personal computers, and in particular for portable personal computers. This memory may be erased and reprogrammed without being removed from the computer by running an update program when the BIOS processes are to be changed. However, in order to erase and reprogram flash EEPROM memory, approximately twelve volts, a voltage level not readily available from the lower voltage batteries provided in portable computers, is required.
Flash EEPROM memory arrays have been used in personal computers as a type of long term memory. A flash EEPROM memory array can be used in place of a hard disk drive, thereby providing a smaller, lighter long term storage device that is not as sensitive to physical damage. Such flash memory arrays are especially useful in portable computers, where space and weight are important considerations. However, these flash EEPROM memory arrays also require much higher voltages and substantially more power than that directly available from the batteries of low power portable computers.
In some electronic devices, charge pump circuits have been used to provide a high voltage from a lower voltage source. Recently, charge pumps have been integrated with flash arrays to supply voltages needed to accomplish erase and program operations of flash memory when such voltages are not available from an external source.
However, erasing and programming the cells of a flash EEPROM memory array requires very accurate voltages. The voltages provided by charge pumps and other circuitry implemented as part of integrated circuits, including voltage reference circuits, typically vary due to factors such as operating temperatures, supply voltages, process variations, and load currents. Moreover, when charge pumps are used to supply the voltage, the voltage level provided at the output terminal of a charge pump tends to vary substantially from a desired voltage value. Reference voltages can be used to regulate the output voltage of charge pumps.
One of the problems encountered in utilizing charge pumps is that the output of a charge pump is furnished in a series of charge pulses which are stored to provide a source voltage. The charge pump generates this form of output because the output stage functions in the manner of a diode which only transfers charge to the output in the forward biased condition. This creates a voltage ripple on the output of the pump when the current requirements are high relative to the capacitance of the load. Another drawback is that power supply noise can be created because the inductance of bond wires and lead frames produces spikes in the internal supply voltage with the pulsing output current provided by the charge pump. The variation in the voltage Vcc provided may be sufficient to cause internal switching of devices. These two different problems make it desirable to attempt to smooth the output voltage provided by the charge pumps.